CN110672495A - Cement-based material moisture permeability prediction method based on low-field magnetic resonance technology - Google Patents
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- 238000004458 analytical method Methods 0.000 description 5
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 4
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- G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
- G01N15/08—Investigating permeability, pore-volume, or surface area of porous materials
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N24/00—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects
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Abstract
The invention discloses a method for predicting the water permeability of a cement-based material based on a low-field magnetic resonance technology, relates to the technical field of cement-based material testing, and aims to provide a method for accurately measuring the water permeability of the cement-based material, which comprises the following steps: the method comprises the following steps: measuring the transverse relaxation time spectrum of the saturated cement-based material test piece by using a low-field magnetic resonance relaxation technology; step two: preparing a cement-based material test piece with a surface adsorbing a single-layer water molecular film by using an isothermal adsorption/desorption balance method; step three: measuring the surface relaxation rate of a single-layer water film in a cement-based material test piece by using a low-field magnetic resonance pulse echo technology; step four: combining the transverse relaxation time spectrum and the surface relaxation rate to obtain a pore size distribution curve of the cement-based material in a water-saturated state; step five: and predicting the water permeability of the cement-based material based on the pore size distribution curve of the cement-based material in a water-saturated state. The method can be used for more accurately predicting the water permeability of the cement-based material.
Description
Technical Field
The invention relates to the technical field of cement-based material testing, in particular to a method for predicting the water permeability of a cement-based material based on a low-field magnetic resonance technology.
Background
Durability of concrete materials to the safety of the concrete structure throughout its life cycleEconomic significance is achieved. How to correctly evaluate and detect the durability of the concrete material is very important for the engineering application of the concrete material. The durability of concrete materials is closely related to how fast aggressive media migrate through the pores inside the material to the interior. The durability problems caused by carbon dioxide gas, chloride ions, sulfate ions, freeze-thaw damage and the like are closely related to the pore structure and the compactness of the cement-based material. Both theory and experimental research show that the moisture permeability is the most important basic index for representing the durability of the concrete material. The water permeability of the cement-based material is extremely low, and the water permeability of the common concrete material can be as low as 1.0 multiplied by 10-20m2The water permeability is 4 to 6 orders of magnitude lower than that of common rock materials, and the permeability of high-performance concrete is lower. Therefore, when the moisture permeability of the cement-based material is directly tested, a very high pressure gradient is required to be applied, which is very strict on the side sealing performance of the test piece. In addition, under the condition of extremely low moisture permeability, the time for the moisture permeation process to reach measurable steady-state seepage is very long, and the measurement precision of steady-state flow is difficult to guarantee, so that the measurement error of the moisture permeability of the cement-based material is large. The method for quickly and accurately predicting the extremely low water permeability of the cement-based material has important significance for quickly representing the pore structure and the durability of the cement-based material.
Since the moisture permeability of cementitious materials is critical in characterizing their durability, there have been numerous related studies devoted to directly predicting the moisture permeability of cementitious materials using pore structure information. The Garboczi contrasts and analyzes the analysis theory of the relation between the pore structure and the permeability, and considers that the Katz-Thompson prediction model of the water permeability is effective based on the characteristic parameters of the pore structure, but the characterization method of the pore structure test is not definitely provided and the relevant analysis model is verified. El-Dieb finds that the correlation between the permeability calculated based on the pore structure obtained by mercury intrusion measurement and the experimental actual measurement result is poor by comparing the experimental actual measurement value of the moisture permeability with the theoretical value calculated based on the pore size distribution, and further considers that the Katz-Thompson theoretical model is not suitable for cement-based materials. Christensen corrects the expression of pore structure construction factors by using the actual measurement result of conductivity on the basis of El-Dieb, and further obtains the water permeability through calculation according to the pore size distribution curve, the correlation of the theoretical value is improved, but the theoretical value is still 2 orders of magnitude higher than the actual measurement result. Tumidajski also researches the calculation and measurement method of the porosity connectivity coefficient, but the result obtained by theoretical prediction based on mercury intrusion porosimetry data is still more than 2 orders of magnitude larger than that of actual measurement data. Cui is analyzed by using an effective medium theory of the composite material, the contribution of capillary holes and gel holes of the cement-based material to permeability is mainly considered, the theoretical calculation precision is improved, but is still unsatisfactory, and the effective medium theoretical model and the parameters thereof are unprecedentedly complex, so that the engineering application is very inconvenient. The moisture permeability of porous media (such as rocks and the like) is determined by a pore structure, and the prediction of the moisture permeability by utilizing pore structure information is completely feasible in theory, but no effective test method or prediction model is suitable for cement-based materials so far.
Because the testing and analysis of the water permeability of the cement-based material are very difficult, a plurality of researches and engineering practices try to firstly dry the cement-based material and then test the gas permeability of the dry cement-based material by using gas, and further use the gas permeability to represent the durability of the cement-based material. Theoretically, the intrinsic permeability obtained by correcting the permeability by considering the difference of the density and the dynamic viscosity of gas and water and the slip effect is independent of the type of fluid. However, intrinsic permeability, measured with inert fluids such as gas and isopropyl alcohol, is still 2-4 orders of magnitude higher than moisture permeability. It is considered that since the drying process of the cement-based material inevitably introduces microcracks, microcracks having a significantly higher permeation rate than pores will cause gas permeability to be significantly higher than moisture permeability, but the reason cannot explain why the gas permeability is still very close to that after pore water is replaced with an organic solvent such as isopropyl alcohol, even though the cement-based material has not undergone drying, and thus has no microcracks. The water permeability of the cement-based material is gradually reduced along with the testing time by the aid of cement hydration, microcrack self-repairing and self-closing effects, but the water permeability of the mature complete cement-based material without drying is still smaller than that of isopropanol by more than 1 order of magnitude. In addition, it is considered that the expansion of cement hydration products caused by contact with water may be the reason that the water permeability is significantly smaller than the gas permeability, but the support of experimental measured data and theoretical analysis models is not available. Recent research suggests that the main composition of the cement-based material, namely hydrated calcium silicate gel, has the characteristic of remarkable water sensitivity of dry shrinkage and wet expansion, which is one of the intrinsic properties of the cement-based material, so that the pore structure of the cement-based material in a saturated state and a dry state is remarkably different, and the difference between the water permeability and the gas and isopropanol permeability is 2 to 4 orders of magnitude. Predicting the permeability based on pore structure data from mercury intrusion tests will be closer to the permeability of gases measured in the dry state, and it is not possible to accurately predict the moisture permeability. Unfortunately, the conventional mercury porosimetry and BET nitrogen adsorption methods are only suitable for pore structure testing in a dry state, and methods capable of accurately testing the pore structure of a cement-based material in a saturated state are very scarce so far. Therefore, how to quickly measure the pore structure of the cement-based material in a water-saturated state and establish a method for analyzing and predicting the water permeability of the cement-based material according to the pore structure has important significance for the characterization and quantitative analysis of the durability of the cement-based material.
Disclosure of Invention
The purpose of the invention is: in view of the above problems, the present invention aims to provide a method for accurately measuring the water permeability of a cement-based material.
The technical scheme adopted by the invention to solve the technical problems is as follows:
a method for predicting the water permeability of a cement-based material based on a low-field magnetic resonance technology comprises the following steps:
the method comprises the following steps: measuring the transverse relaxation time spectrum of the saturated cement-based material test piece by using a low-field magnetic resonance relaxation technology;
step two: preparing a cement-based material test piece with a surface adsorbing a single-layer water molecular film by using an isothermal adsorption/desorption balance method;
step three: measuring the surface relaxation rate of a single-layer water film in a cement-based material test piece by using a low-field magnetic resonance pulse echo technology;
step four: combining the transverse relaxation time spectrum and the surface relaxation rate to obtain a pore size distribution curve of the cement-based material in a water-saturated state;
step five: and predicting the water permeability of the cement-based material based on the pore size distribution curve of the cement-based material in a water-saturated state.
Further, the concrete steps of the fifth step are as follows: first of all, the surface relaxation rate ρ is used2Transverse relaxation time spectrum fi(T2i) Converted into cement-based material with pore size distribution curve f in water-saturated statei(ri) Wherein r isi＝2ρ2T2iThen, based on the pore size distribution curve of the cement-based material in the water saturation state, predicting the water permeability of the cement-based material in the water saturation state by utilizing a Katz-Thompson model or a Carmen-Kozeny model, wherein riIs an equivalent radius, ρ2As surface relaxation rate, T2iAs surface relaxation time, fiIs the pore volume fraction.
Further, the concrete steps of the fifth step are as follows:
first of all, the surface relaxation rate ρ is used2Transverse relaxation time spectrum fi(T2i) Converted into cement-based material with pore size distribution curve f in water-saturated statei(ri) Wherein r isi＝2ρ2T2iThen, the characteristic pore diameter where the slope of the pore size distribution curve has the maximum negative value is taken as the percolation pore diameter rthI.e. the maximum characteristic pore diameter of the continuous path formed by water in the pores in the water-saturated state, and finally based on the percolation pore diameter rthAnd predicting the water permeability of the cement-based material in a water-saturated state by using a Katz-Thompson theoretical formula.
Further, the moisture permeability k is predicted by the following formula:
where α is a constant, α is 1/226, F is the formation factor, Φ is the total porosity, s (x) denotes a pore diameter greater thanVolume fraction of all pores of x, rthIs the percolation pore size.
Further, the transverse relaxation time spectrum is obtained by the following steps:
firstly, saturating a test piece with water by using a vacuum water saturation method, then testing by using a low-field magnetic resonance instrument and a CPMG pulse sequence, and finally obtaining a transverse relaxation spectrum f of the test piece by using an inverse Laplace transform inversion algorithmi(T2i)。
Further, the main parameters of the test are resonance frequency, echo interval, scanning times, waiting time and echo number.
Further, the surface relaxation rate is obtained by the following steps: firstly, making the internal pores of the test piece adsorb a single-layer water molecular film and reach an equilibrium state, then testing by using a low-field magnetic resonance instrument and a Hahn pulse echo sequence, and finally performing linear fitting on the echo intensity and the echo interval to obtain the surface relaxation rate rho2＝λ/T2Sλ is the diameter of water molecule, λ is 0.3 nm.
Further, the formula of the linear fitting is as follows:
where M (t) is the echo intensity at an echo interval of t, M0For initial echo intensity, T2SThe surface relaxation time.
The invention has the beneficial effects that:
(1) the method takes water in the hole as a probe, and can be well suitable for measuring the pore size distribution curve in a water-saturated state;
(2) the CPMG sequence and the Hahn pulse echo sequence are utilized to measure the cement-based material test piece, the test is very quick and accurate, the test is not damaged on the test piece, and the repeated test is convenient for a plurality of times;
(3) by reasonably selecting related parameters of low-field magnetic resonance test, all evaporable water in the cement-based material can be accurately detected, and further, the porosity and tortuosity coefficient of the cement-based material can be accurately tested;
(4) the test piece used in the low-field magnetic resonance test is far larger than the test pieces used in the mercury intrusion method and the BET nitrogen adsorption method, and the representativeness of the result measured by the method is very strong in consideration of the inherent heterogeneity of the cement-based material;
(5) because the principle of testing the pore structure by the low-field magnetic resonance technology is obviously different from the testing principle of pressing non-wetting mercury into a dry porous material by a mercury pressing method, the percolation pore diameter in a water-saturated state is accurately identified by adopting the adjusted identification standard, the water permeability of the cement-based material can be more accurately predicted, the precision is higher than that of all theoretical prediction methods in documents, and the engineering application is very convenient. The method can directly, quickly, nondestructively and accurately test the porosity and the pore size distribution curve of the water saturation test piece, can accurately predict the extremely low water permeability of the cement-based material, is convenient and quick in test and test, high in prediction precision and good in result reproducibility, and can be applied to the fields of quick test of the pore structure and permeability of the cement-based material and detection and analysis of the durability of actual engineering.
Drawings
Fig. 1 is a transverse relaxation time spectrum of a white cement mortar test piece.
FIG. 2 is a graph of results of Hahn pulse echo test and linear fitting of a white cement mortar test piece.
FIG. 3 is a graph of equivalent pore size distribution and percolation pore size identification for a saturated white cement mortar material.
Fig. 4 is a comparison graph of the measured value and the theoretical predicted value of the water permeability of the white cement mortar test piece.
Detailed Description
The first embodiment is as follows: specifically describing the present embodiment with reference to fig. 1 to 4, the method for predicting the moisture permeability of a cement-based material based on a low-field magnetic resonance technology in the present embodiment includes the following steps:
the method comprises the following steps: measuring the transverse relaxation time spectrum of the saturated cement-based material test piece by using a low-field magnetic resonance relaxation technology;
step two: preparing a cement-based material test piece with a surface adsorbing a single-layer water molecular film by using an isothermal adsorption/desorption balance method;
step three: measuring the surface relaxation rate of a single-layer water film in a cement-based material test piece by using a low-field magnetic resonance pulse echo technology;
step four: combining the transverse relaxation time spectrum and the surface relaxation rate to obtain a pore size distribution curve of the cement-based material in a water-saturated state;
step five: predicting the water permeability based on the pore size distribution curve of the cement-based material in the water-saturated state
The second embodiment is as follows: this embodiment is a further description of the first embodiment, and the difference between this embodiment and the first embodiment is that the specific step of the fifth step is: first of all, the surface relaxation rate ρ is used2Transverse relaxation time spectrum fi(T2i) Converted into cement-based material with pore size distribution curve f in water-saturated statei(ri) Wherein r isi＝2ρ2T2iThen, based on the pore size distribution curve of the cement-based material in the water saturation state, predicting the water permeability of the cement-based material in the water saturation state by utilizing a Katz-Thompson model or a Carmen-Kozeny model, wherein riIs an equivalent radius, ρ2As surface relaxation rate, T2iAs surface relaxation time, fiIs the pore volume fraction.
The third concrete implementation mode: this embodiment is a further description of the first embodiment, and the difference between this embodiment and the first embodiment is that the specific step of the fifth step is:
first of all, the surface relaxation rate ρ is used2Transverse relaxation time spectrum fi(T2i) Converted into cement-based material with pore size distribution curve f in water-saturated statei(ri) Wherein r isi＝2ρ2T2iThen, the characteristic pore diameter where the slope of the pore size distribution curve has the maximum negative value is taken as the percolation pore diameter rthI.e. the maximum characteristic pore diameter of the continuous path formed by water in the pores in the water-saturated state, and finally based on the percolation pore diameter rthAnd predicting the water permeability of the cement-based material in a water-saturated state by using a Katz-Thompson theoretical formula.
The fourth concrete implementation mode: this embodiment is a further description of a third embodiment, and is different from the third embodiment in that the moisture permeability k is predicted by the following formula:
where α is a constant, α is 1/226, F is the formation factor, Φ is the total porosity, s (x) represents the volume fraction of all pores with a pore diameter greater than x, r isthIs the percolation pore size.
The fifth concrete implementation mode: this embodiment mode is a further description of the first embodiment mode, and is different from the first embodiment mode in that the transverse relaxation time spectrum is obtained by the following steps:
firstly, saturating a test piece with water by using a vacuum water saturation method, then testing by using a low-field magnetic resonance instrument and a CPMG pulse sequence, and finally obtaining a transverse relaxation time spectrum f of the test piece by using an inverse Laplace transform inversion algorithmi(T2i)。
The sixth specific implementation mode: the present embodiment is a further description of a fifth embodiment, and the difference between the present embodiment and the fifth embodiment is that the main parameters of the test are the resonance frequency, the echo interval, the number of scans, the waiting time, and the number of echoes.
The seventh embodiment: this embodiment mode is a further description of the first embodiment mode, and is different from the first embodiment mode in that the surface relaxation rate is obtained by the following steps: firstly, making the internal pores of the test piece adsorb a single-layer water molecular film and reach an equilibrium state, then testing by using a low-field magnetic resonance instrument and a Hahn pulse echo sequence, and finally performing linear fitting on the echo intensity and the echo interval to obtain the surface relaxation rate rho2＝λ/T2SAnd λ is the water molecule diameter (0.3 nm).
The specific implementation mode is eight: this embodiment is a further description of a seventh embodiment, and the difference between this embodiment and the seventh embodiment is that the linear fitting formula is:
where M (t) is the echo intensity at an echo interval of t, M0For initial echo intensity, T2SThe surface relaxation time.
Example (b):
(1) white cement and river sand are adopted to prepare white cement mortar with the ash-sand ratio of 1:3, but the water-ash ratio is respectively 0.35, 0.40, 0.45, 0.50 and 0.55, the white cement mortar is maintained in 20-degree water for 1 month after the mould is removed, and then a small round cake test piece with the diameter of 25mm and the height of 25mm is prepared by drilling a core and is used for testing and measuring the water permeability of the small round cake test piece; meanwhile, a small cylindrical test piece with the diameter of 25mm and the height of 50mm is prepared by drilling the core and is used for low-field magnetic resonance testing. All the white cement mortar test pieces were first cured in 80 degrees hot water for 2 months to completely hydrate the cement, and then subsequently tested after curing in 20 degrees water for 3 months (total age is half a year).
(2) The water permeability tester is adopted to test the water permeability of a small round cake test piece with the diameter of 25mm and the height of 25mm, the confining pressure for sealing the side surface of the test piece is set to be 9MPa, and the water inlet pressure P of the bottom surface of the test pieceiIs 3MPa, the top face is facing the atmosphere (pressure P)0Atmospheric pressure), the water flow passing through the interior of the test piece is monitored for 7 days continuously under the action of constant pressure gradient, and after the water flow reaches a steady state, the water flow Q (m) is calculated according to the relation between the water inflow and the test piece3S) and further calculating the water permeability k (m) according to Darcy's law2)：
Wherein mu is the dynamic viscosity coefficient of water, L is the length of the test piece, and A is the cross-sectional area of the test piece.
(3) Saturating a white cement mortar test piece with the diameter of 25mm and the height of about 50mm by using a vacuum water saturating method, wrapping the test piece by using a preservative film to prevent the test piece from losing water, and then putting a low-field magnetic resonance with the dominant frequency of 2MHz of Beijing Qing lemon echoy science and technology LimitedThe instrument sample chamber is tested by adopting a CPMG pulse sequence, and the main test parameters are as follows: the echo interval is 60 mu s, the scanning times are 64 times, the waiting time is 15s, the echo number is 50-100k, the water in the test piece is completely relaxed, the signal to noise ratio is ensured to be between 80 and 100, the relaxation signal in the water-saturated test piece is obtained by about 30min of instrument test, and the transverse relaxation spectrum f of the test piece is obtained by adopting the inverse Laplace algorithmi(T2i) As shown in fig. 1 below.
(4) Placing the test piece with the diameter of 25mm and the height of 50mm of each white cement mortar material into a closed container with the internal relative humidity controlled by saturated potassium acetate solution being constant at 23%, monitoring the mass of the test piece every 7 days, and carrying out magnetic resonance test until the mass change of the test piece every 7 days is less than 0.1%. Wrapping the test piece in equilibrium state with preservative film, testing with 2MHz low-field magnetic resonance instrument of Beijing green lemon echocardiogram technology, and performing linear fitting on echo intensity and echo interval by using Hahn pulse echo sequence with echo interval of 60 μ s, 70 μ s and 80 μ s … … 120 μ s
Further calculating to obtain the surface relaxation time T2SAnd surface relaxation rate ρ2＝λ/T2S. Typical test results are shown in fig. 2 below. A plurality of test pieces were tested to obtain an average value of the surface relaxation rates of 1.69 μm/s.
(5) Using surface relaxation rate p2Transverse relaxation time spectrum fi(T2i) Conversion into pore size distribution curve fi(ri) As shown in fig. 3, this is to measure the pore size distribution of the obtained cement-based material in a water-saturated state by using a low-field magnetic resonance technique.
(6) Identifying the aperture with the largest df/dlog (r) negative value on a logarithmic graph by using a computer program, namely measuring the percolation aperture r of the pore structure of the obtained cement-based material in a water-saturated state by using a low-field magnetic resonance technologyth. The percolation pore diameter corresponding to the pore diameter distribution curve measured in the embodimentAs shown in fig. 3.
(7) Based on the pore size distribution curve in the water saturation state and the identified percolation pore size shown in FIG. 3, the water permeability k of each test piece can be calculated by using the classical Katz-Thompson theory,
wherein, alpha is 1/226, F is a construction factor, phi is total porosity, and the total porosity can be obtained by calculation according to equivalent water content converted from semaphore obtained by a magnetic resonance test water saturation test piece; s (x) represents the volume fraction of all pores with pore diameters greater than x. All variables in this equation can be calculated from the pore size distribution curve shown in FIG. 3 and the identified percolation pore size. In this embodiment, the comparison of the water permeability calculated from the pore size distribution curve and the percolation pore size with the experimental measurements is shown in fig. 4 below. It can be seen from the figure that the relative error between the predicted value and the measured value of the moisture permeability calculated based on the pore size distribution curve and the percolation pore size in the water saturation state and the Katz-Thompson theoretical formula is in the range of [ -18.8% and 26.1% ], the test error range of the moisture permeability is reached, and the precision is far higher than that of the related method reported in the literature.
(8) If the prediction is not based on the percolation pore diameter and the Katz-Thompson theory, the prediction can be carried out by using models such as Carmen-Kozeny and the like only using a pore diameter distribution curve, and the prediction is also within the protection scope of the patent right.
It should be noted that the detailed description is only for explaining and explaining the technical solution of the present invention, and the scope of protection of the claims is not limited thereby. It is intended that all such modifications and variations be included within the scope of the invention as defined in the following claims and the description.
Claims (8)
1. A method for predicting the water permeability of a cement-based material based on a low-field magnetic resonance technology is characterized by comprising the following steps of:
the method comprises the following steps: measuring the transverse relaxation time spectrum of the saturated cement-based material test piece by using a low-field magnetic resonance relaxation technology;
step two: preparing a cement-based material test piece with a surface adsorbing a single-layer water molecular film by using an isothermal adsorption/desorption balance method;
step three: measuring the surface relaxation rate of a single-layer water film in a cement-based material test piece by using a low-field magnetic resonance pulse echo technology;
step four: combining the transverse relaxation time spectrum and the surface relaxation rate to obtain a pore size distribution curve of the cement-based material in a water-saturated state;
step five: and predicting the water permeability of the cement-based material based on the pore size distribution curve of the cement-based material in a water-saturated state.
2. The method for predicting the water permeability of the cement-based material based on the low-field magnetic resonance technology according to claim 1, wherein the concrete steps of the fifth step are as follows: first of all, the surface relaxation rate ρ is used2Transverse relaxation time spectrum fi(T2i) Converted into cement-based material with pore size distribution curve f in water-saturated statei(ri) Wherein r isi＝2ρ2T2iThen, based on the pore size distribution curve of the cement-based material in the water saturation state, predicting the water permeability of the cement-based material in the water saturation state by utilizing a Katz-Thompson model or a Carmen-Kozeny model, wherein riIs an equivalent radius, ρ2As surface relaxation rate, T2iFor transverse relaxation time, fiIs the pore volume fraction.
3. The method for predicting the water permeability of the cement-based material based on the low-field magnetic resonance technology according to claim 1, wherein the concrete steps of the fifth step are as follows:
first of all, the surface relaxation rate ρ is used2Transverse relaxation time spectrum fi(T2i) Converted into cement-based material with pore size distribution curve f in water-saturated statei(ri) Wherein r isi＝2ρ2T2iThen, the position where the slope of the pore diameter distribution curve is maximum is processedThe characteristic pore diameter of (A) is regarded as the percolation pore diameter rthI.e. the maximum characteristic pore diameter of the continuous path formed by water in the pores in the water-saturated state, and finally based on the percolation pore diameter rthAnd predicting the water permeability of the cement-based material in a water-saturated state by using a Katz-Thompson theoretical formula.
4. The method for predicting the water permeability of the cement-based material based on the low-field magnetic resonance technology as claimed in claim 3, wherein the water permeability k is predicted by the following formula:
F＝0.34φS(0.34rth)
where α is a constant, α is 1/226, F is the formation factor, Φ is the total porosity, s (x) represents the volume fraction of all pores with a pore diameter greater than x, r isthIs the percolation pore size.
5. The method for predicting the water permeability of the cement-based material based on the low-field magnetic resonance technology according to claim 1, wherein the transverse relaxation time spectrum is obtained by the following steps:
firstly, saturating a test piece with water by using a vacuum water saturation method, then testing by using a low-field magnetic resonance instrument and a CPMG pulse sequence, and finally obtaining a transverse relaxation time spectrum f of the test piece by using an inverse Laplace transform inversion algorithmi(T2i)。
6. The method for predicting the water permeability of the cement-based material based on the low-field magnetic resonance technology as claimed in claim 5, wherein the main parameters of the test are resonance frequency, echo interval, scanning times, waiting time and echo number.
7. The method for predicting the water permeability of the cement-based material based on the low-field magnetic resonance technology according to claim 1, wherein the surface relaxation rate is obtained by the following steps: firstly, makeAbsorbing a single-layer water molecular film by pores in the test piece to reach an equilibrium state, testing by using a low-field magnetic resonance instrument and a Hahn pulse echo sequence, and finally performing linear fitting on the echo intensity and the echo interval to obtain the surface relaxation rate rho2＝λ/T2Sλ is the diameter of water molecule, λ is 0.3 nm.
8. The method for predicting the water permeability of the cement-based material based on the low-field magnetic resonance technology as claimed in claim 7, wherein the linear fitting formula is as follows:
where M (t) is the echo intensity at an echo interval of t, M0For initial echo intensity, T2SThe surface relaxation time.
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